Niemann Pick disease type C (NPC) is an autosomal recessive lipidosis that is characterized by lysosomal storage of cholesterol and glycosphingolipids. NPC patients suffer from cholestasis, prolonged jaundice, and hepatosplenomegaly. The mechanism by which NPC1 protein dysfunction leads to liver disease is unknown. We have developed a unique mouse model with which to study NPC1 liver disease. Treatment of mice with a NPC1 specific antisense oligonucleotide leads to liver specific and reversible knockdown of NPC1 protein expression. Our goal is to determine the mechanism by which lysosomal lipid storage leads to liver disease. Our hypothesis is that NPC liver disease is initiated by the massive lipid storage, which leads to lysosome destabilization and release of pro-apoptotic proteases and lipids. Hepatocyte apoptosis is then propagated through the TNFa pathway. We expect that if NPC1 were re-expressed in the knockdown mouse, then the fibrotic liver would recover. Because our model of NPC1 knockdown is reversible, we have the ability to study disease regression. Specific Aim #1 - To determine if the lysosomal lipid storage in NPC mouse hepatocytes leads to rupture of lysosomes and release of their contents to activate apoptosis. Our preliminary results suggest that NPC1 knockdown leads to release of cathepsins from lysosomes. We will determine if cathepsin knockdown reduces NPC hepatocyte apoptosis. Specific Aim #2 - To determine if NPC liver disease is propagated through the TNFa pathway. Our preliminary results indicate that NPC1 knockdown in TNFa-deficient mice leads to a less severe disease phenotype. We will test this hypothesis using TNF1 knockout mice, liver- specific knockdown of the TNFa-receptor, TNF-RI, and an inhibitor of the TNFa pathway. Specific Aim #3 - To determine the course of reversal of NPC liver disease upon re-expression of the NPC1 protein in the NPC1 knockdown mouse model. Our preliminary results indicate that NPC1 re-expression leads to reduced liver injury and inflammation. We will determine the extent to which NPC1 re-expression fully reverses the hepatic disease phenotype. PUBLIC HEALTH RELEVANCE: Our studies will identify the signals that initiate and propagate Niemann-Pick C liver disease, providing information that is critical for designing therapeutic strategies. We will also determine the extent to which NPC liver disease can be reversed in order to achieve maximal benefit from an NPC therapy.